I've saw how to reference items and skills with the [ ] [/] in descriptions. How you give reference, or internal links, to feats? Like in the Witherbloom Student background for example. In the Witherbloom Initiate description, they linked the Strixhaven Initiate feat to the text. I tried using the Insert/edit Link tool, but it meant that I couldn't publish due to an external link. Any tips? Am I making sense?

No. Those [ ] [/] links are called tooltips. There are no tooltips for feats or backgrounds, only monsters, spells, magic items, and basic equipment. The links you see in the Strixhaven backgrounds are actual hyperlinks, and as you’ve noticed, inserting hyperlinks into a homebrew makes it unpublishable. The good news is you don’t have to publish your homebrews for you and your friends to use them. Publishing is only so that perfect strangers can use them.

Thank you for the quick response. It was very helpful.

Happy to help 

Thanks for the help. Is there a name for the {{ }} brackets you use to perform calculations in snippets?

You mean the snippets of code enclosed by the double brackets, or the brackets themselves? Because the brackets are just called “brackets,” and the snippets of code are actually called “snippets.” The snippet fields were named after them because that’s where they function.